(a) Field of the Invention
The present invention relates to a charge-coupled device (CCD) having a reduced width for barrier sections in a transfer channel and thus suited to a finer structure of the CCD.
(b) Description of the Related Art
In a recent solid-state imaging device including a CCD, the dimensions of the pixels are more and more reduced to have a finer structure, whereby the CCD therein is also requested to have a smaller width. The width of the CCD is an important factor which determines the amount of electrons (or electric charge) to be transferred in the solid-state imaging device, and a larger width of the CCD allows the CCD to transfer a lager amount of electrons therein and to afford an improved image quality for the solid-state imaging device.
FIG. 14 shows the structure of transfer electrodes in a conventional CCD in a top plan view. The CCD includes a plurality of first-level transfer electrodes 11 and a plurality of second-level transfer electrodes 12, which are alternately arranged along a transfer channel 17 to transfer electric charge within the transfer channel 17. The transfer channel 17 includes therein an n-well 20 heavily doped with n-type impurities and underlying the electrodes 11 and 12. A first group of pairs each including one of the first-level transfer electrodes 11 and an adjacent one of the second-level transfer electrodes 12 and a second group of pairs each including another of the first-level transfer electrodes 11 and adjacent another of the second-level transfer electrodes 12 are alternately arranged along the transfer channel 17. The first group of pairs are connected to a first interconnect line, whereas the second group of pairs are connected to a second interconnect line.
FIG. 15 shows the underlying transfer channel 17 in a top plan view. The transfer channel 17 is encircled by a p-well, and includes an n-well 20 heavily doped with impurities and a plurality of stripe nxe2x88x92-wells 21 lightly doped with impurities and arranged along the transfer channel 17. The nxe2x88x92-wells 21 are formed on the surface regions of the n-well 20. The portions of the n-well 20 exposed from the nxe2x88x92-wells 21 underlie the first-level transfer electrodes 11, whereas the nxe2x88x92-wells 20 underlie the second-level transfer electrodes 12. The electric charge is transferred along the transfer channel 17 in the direction of the arrows depicted.
FIG. 16 shows a flowchart of the process for manufacturing the CCD shown in FIG. 14. The n-well 20 of the transfer channel 17 is first formed within a p-well formed in a semiconductor substrate (step S1), followed by implantation of boron ions into the peripheral area of the transfer channel 17 to form the p+-diffused region (step S2). Thereafter, an oxide film is formed over the entire surface of the substrate (step S3), followed by depositing a first polysilicon film and patterning thereof to thereby form the first-level transfer electrodes 11 (step S4). Subsequently, boron ions are implanted into surface regions of the n-well 20 in a self-alignment technique using the first-level transfer electrodes 11 as a mask, thereby selectively changing the surface regions of the n-well 21 to the nxe2x88x92-wells 21 (step S5). Thereafter, an oxide film and an inter-level dielectric film are formed (step S6), followed by depositing a second polysilicon film and patterning thereof to form the second-level transfer electrodes 12 (step S7).
As shown in FIG. 14, it is assumed that P, S1, S2, A1, A2, A3, A4 and A5 are pitch of the combination transfer electrodes 11 and 12, space between adjacent two first-level transfer electrodes 11, space between adjacent two second-level transfer electrodes 12, distance between the contact plug 13 and the edge of the corresponding first-level transfer electrode 11, distance between the contact plug 13 and the edge of the corresponding second-level transfer electrode 12, width of the first-level transfer electrodes 11, dimension of the overlapped portion between the first-level transfer electrode 11 and the corresponding second-level transfer electrode 12 and width of the contact plugs 13, respectively.
In the design of the CCD shown in FIG. 14, the above pitch P, spaces S1 and S2, distances A1 and A2, width A3, dimension A4 and width A5 are determined in consideration of the design margin so that the pitch P satisfies the following relationship:
Pxe2x89xa7S1+S2+A1+A2+A4+A5.
This relationship, if satisfied, allows the CCD to have the overall configuration shown in FIG. 14. However, due to the recent development of smaller dimensions for the pixels of CCD, it is desired that the pitch P of the combination transfer electrodes be equal to or below 2 xcexcm, which fact renders the employment of configuration shown in FIG. 14 to be difficult.
It may be considered that such a small-dimension CCD should have the configuration shown in FIG. 17 and FIG. 18, which show the structure of the CCD similarly to FIGS. 14 and 15, respectively. In the depicted structure, the contact plugs 13 connecting the first interconnect line 41 and the corresponding transfer electrodes 11 and 12 in the first group are disposed in the vicinity of one edge of the transfer channel 17 opposite to the edge, in the vicinity of which the contact plugs 13 connecting the second interconnect line 42 and the corresponding transfer electrodes 11 and 12 in the second group are disposed. In other words, the contact plugs 13 are arranged in a staggered configuration with respect to the center of the transfer channel 17. This structure may allow the design margin in the patterning for the contact plugs 13 to be reduced to reduce the pitch P of the combination transfer electrodes. However, this structure has a disadvantage in that the width (W2) of the transfer channel 17 is reduced, as shown in FIGS. 18 and 19, whereby the maximum electric charge to be transferred by the transfer channel 17 is also reduced.
In order to assure a sufficient width for the transfer channel 17, another structure such as shown in FIG. 19 may be considered. However, this structure requires a sufficient space between adjacent two second-level transfer electrodes 12 for assuring an equal width for the second-level transfer electrode 12 and the barrier section or nxe2x88x92-well. This results in a larger pitch P for the transfer channel, and thus is not suitable.
In view of the above, it is an object of the present invention to provide a CCD having a reduced pitch P for the combination transfer electrodes and a width sufficient for transferring an adequate amount of electric charge, irrespective of the CCD being designed in a design rule similar to the conventional design rule and manufactured by a process similar to the conventional process.
The present invention provides a charge-coupled device (CCD) including: a semiconductor substrate having therein a transfer channel on a surface region of the semiconductor substrate; a plurality of first transfer electrodes and a plurality of second transfer electrodes overlying the semiconductor substrate and alternately arranged along the transfer channel; and first and second interconnect lines for supplying two-phase driving signals to the first and second transfer electrodes to transfer electric charge along the transfer channel, wherein: the transfer channel includes a plurality of first diffused regions each underlying a corresponding one of the first transfer electrodes and a plurality second diffused regions each underlying a corresponding one of the second transfer electrodes, the first diffused regions constituting charge storage sections and the second diffused regions constituting barrier sections during transferring the electric charge; and each of the charge storage sections has a width larger than a width of each of the barrier sections.
In accordance with the CCD of the present invention, by allowing the width of the barrier sections to be smaller than the width of the charge storage sections, the second transfer electrodes overlying the barrier sections may have a smaller width whereby the pitch of the combination transfer electrodes can be reduced in the staggered arrangement of the contact plugs. The smaller width of the barrier sections does not substantially reduce the maximum amount of electric charge transferred by the transfer channel because the maximum amount is determined by the width of the charge storage sections and scarcely by the width of the barrier sections.
The present invention also provides a method for manufacturing the CCD of the present invention, the method including the step of selectively implanting impurities in the charge storage section in a self-alignment technique using the first transfer electrodes as a mask to form the barrier sections.
In accordance with the method of the present invention, the self-alignment technique using the transistor electrodes reduces the number of photolithographic steps in the manufacture of the CCD while assuring an accurate selectivity.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.